Residual strain is developed within a composite part with the elevation in temperature and the chemical reactions which the composite part experiences during the curing process. Residual strain retained within a cured composite part facilitates cracking of the composite material and affects fatigue performance of the composite part. Understanding the amount of residual strain retained within various locations within the composite part provides useful information for a designer and/or fabricator in providing them an opportunity to reduce the cracking phenomenon and improve the fatigue performance of the composite part.
Existing methods are used to measure residual strain in an isotropic plastic material which is not a composite material which includes fibers. Thermal strains in composite parts have been measured by measuring the change of strain by increasing the temperature of the composite part to the cure temperature. This methodology however does not measure the residual strain but only measures strain imparted to the composite part up to the cure temperature.
There is a method employed for measuring residual strains in isotropic material which includes hole-drilling into the part and installation of a strain gage that measures residual strain at the surface of the part. The use of hole-drilling limits the geometry of material removal which in turn limits the application to a low-gradient strain field. If a complex, high gradient strain field is of interest, the removal process must involve geometries much more complex than a circular hole. The use of a strain gage also limits the spatial resolution of strain measurements as the resolution is limited by the size of the gage and further limited by the occupation of space associated with the part being measured with wiring. In addition, existing methods can only measure limited locations with respect to the part due to the finite amount of strain gages that are employable and cannot be used to measure a full-field of strain distribution.
There is a need for measuring residual strain within a cured composite part which has been retained within the composite part from the cure process wherein a full field, high gradient and multi-axial strain measurements can be taken on the surface of the composite part. Furthermore, there is a need to eliminate geometry restraints imposed by the installation or placement of strain gages. Moreover a method is needed that can readily enable measurement of strain in any target geometry and around any strain gradient field of interest.